Power distribution lines are well known as are dedicated communication lines. In recent years, efforts have been made to combine power and communication lines so as to permit data signals over power distribution lines. Methods of attempting this are well known and typically operate by impressing a high frequency modulated carrier signal onto the alternating current waveform of the power distribution wiring system. The resulting reduction in infrastructure both reduces the cost and effort to provide data services to various locations, but also provides greater modularity to moving or relocating points of need for data transmission as any existing power distribution line may be utilized to provide both of these services simultaneously.
There are however, a number of difficulties to transmitting data over power distribution lines. Most significantly, electrical noise may serve to obscure the data signal transmitted and received from one device to another. Sources of noise may disadvantageously correspond to the data transmission frequency which will therefore obscure or otherwise interfere with the transmission and reception of the desired data signal.
In particular, difficulties have arisen when a plurality of devices are located on a plurality of phases that are required to talk to a central controller. It will be seen that where a plurality of phases are used, the addition of noise from all the phases will prevent devices on the different phases from communicating with a central controller due to unattenuated noise on one phase interfering with an attenuated signal on another phase, herein intended to include where in two phases there is equal attenuation in the two phases with one phase noisier than the other so as to interfere with the less noisy phase, or to the absence of a quiet period during the signal at the zero crossing of the alternating current waveform.
FIG. 1 helps demonstrate the problem that exists. In this figure there are a number of slave units 6 located on first second and third phases 12a, 12b and 12c, respectively of the power line system 10 at a distance from a central controller 8. As well as the slaves 6 there are noise sources. In the system illustrated in FIG. 1, the Central Controller 8 will have difficulty hearing slave units 6 whose signal is obscured by noise.
The first phase 12a of FIG. 1 shows that there is high attenuation on this phase between the slaves 6 and the central controller 8 and between the noise sources on this phase and the central controller 8. The second phase 12b, shows that there is a small amount of attenuation between the slaves 6 and the central controller 8 and between the noise sources and the central controller 8. Because the signals from the slaves 6 and the noise are summed at the central controller 8 the signals from those slaves 8 associated with the first phase 12a, can easily be obscured by the un-attenuated noise from the second phase 12b. 
Illustrated in FIGS. 2a and 2b, are voltage measurements 17 of the signal on a single and three phase system including a noise source as measured by an oscilloscope with the power line AC waveform removed. FIG. 2a show the noise from a halogen lamp power supply over two cycles of a single phase of the power line. As can be seen there is a quiet period 18 when the noise is at a minimum. This period is associated with the zero crossing of the power line when the power supply effectively turns off. Even if the bulk of the noise is at the frequency of communications, signals can be easily received during the quiet period.
FIG. 2b shows the same noise as FIG. 2a but now coming from three halogen power supplies on three different phases and summed. As can be seen the quiet period is no longer present.
Various methods have been attempted to attenuate the noise sources from a data signal. Many of these have required that the attenuating device be inserted into the power distribution line itself or around the line as an RF Choke. Such devices are not desirable due to the fact that devices inserted into the power line need to be sized large enough to handle the power capacity of the power line. RF chokes are also known to be of limited usefulness for high power applications as well as for preventing cross-phase noise. Examples of such systems include U.S. Pat. No. 5,497,142 to Chaffanjon and U.S. Pat. No. 7,102,478 to Pridmore, Jr. et al.
Other attempts to solve this problem have not provided for a method of isolating the separate phases from each other so as to prevent noise from one phase obscuring the signal in another phase. An example of such attempts is illustrated in U.S. Patent Application No. US2004/0108941 to Hauck et al. which utilizes a repeater coupler between phases. Similarly, U.S. Pat. No. 6,317,031 to Rickard relies on coupling between proximate different phases in a common distribution cable such that leakage between the phases permits a data signal on one phase to be induced on proximate phases.
These solutions have not been useful for the present application. As mentioned above, these previous attempts to solve this problem have not provided a method for attenuating the noise between phases. Rather the solutions of Hauck et al. and Rickard will transmit or amplify the data signal and the noise signal of one phase to another. For this reason, both of these devices will not increase the ability of a single device to bear or transmit data to a device on a separate phase as the noise of both phases will add together to eliminate quite periods at the zero crossing of a single phase.
Other solutions have relied upon complex devices or devices in which separate amplifiers or receivers are provided for each phase. Examples of such devices are provided in U.S. Pat. No. 4,382,248 to Pai and U.S. Patent Application Publication No. US2002/0071452 to Abraham. These devices would not be suitable for most applications due to the costs and spatial requirements of such devices.
For the above reasons it is desirable to have a way of separating the various phases. However, the central controller needs to communicate to all the slaves. Often it is the case that the central controller can communicate directly to many or most of the slaves on a given phase. So what is desired is a way of having direct communications between the different phases and the central controller as well as having a way to separate the different phases from each other.